Energy storage devices such as batteries that utilize the oxidation and reduction reactions of an alkali metal are known. Such “lithium ion cells” include secondary cells which use a carbon material that can be doped and undoped with lithium ions as a negative pole and which use a complex oxide of lithium and a second metal as a positive pole. Lithium ion cells are compact and lightweight, and have a high energy density. Accordingly, the use of lithium ion cells as secondary cells in portable electronic devices has expanded rapidly. Concurrently, there has been an escalating demand for improved performance, e. g., an increase in the energy density and an increase in the discharge current, etc., in lithium ion cells in order to achieve a further improvement in the function of such portable electronic devices.
The decreased size of such energy storage devices has resulted in the presence of highly energetic active substances in a small, confined volume. As a result, large amounts of energy can be released when electrodes short-circuit or otherwise fail as a result of, for example, piercing and compression that may cause a battery to ignite and catch fire. As the cell capacity has increased, there has been a strong demand for an improvement in battery safety.
Previous attempts to increase safety have been directed to changing electrode construction or changing the active substance. Other studies have focused on additives to the electrolyte solution that increase the safety of the batteries. Thus, for example phosphorus and fluorine compounds such as triphenyl phosphate and fluoro-ethers have been dissolved in the electrolyte solution to improve battery safety. However, these compounds may be subject to oxidation-reduction by the electrodes, or may react with the electrodes so that the capacity is lowered. Although safety is improved when the amounts of additives are increased, battery performance deteriorates. Accordingly, it has been difficult to realize increased safety without causing a deterioration of conventional battery characteristics.
There is thus a continuing need for improved cell capacity, charge-discharge rate, and charge-discharge cycle.
There is also a continuing need for an improved battery with an improved safety profile that does not deteriorate over time.